In radio communications, compact antenna systems are desirable for reasons such as portability, cost, and ease of manufacture. Interest in compact antenna systems has been further stimulated by the use of higher radio frequencies, for example UHF and higher, which allow for antenna lengths significantly less than 1 centimeter, and by the development of lithographic techniques which allow for antenna systems to be printed directly onto circuit boards with small form factors at low cost. However, due to other limitations, such as limited energy sources, regulations limiting the field strength of radio frequency activity, and limitations on energy flow in radio systems of compact size, such antenna systems are often highly complex if they are to achieve high bandwidth requirements of many radio systems. This complexity often results in a large number of precisely manufactured components, making it challenging to provide an antenna system that is both compact and exhibits the performance required of modern radio systems.
An important factor affecting the performance of an antenna system is the tendency for radio communication to be degraded by undesirable interference. For example, electromagnetic radiation from an antenna may reach its destination through multiple paths, as it is reflected off various surfaces in the environment. Since these paths are of different lengths, electromagnetic radiation due to each path may exhibit destructive interference at the destination, a phenomenon known as multipath interference. One method to combat multipath interference is to transmit or receive over multiple channels using multiple antennas, a strategy known as antenna diversity. Typically, the best channel is then used for communication, thereby increasing performance.
Two well-known methods in the art for providing antenna diversity are known as polarization diversity and pattern diversity. Polarization diversity uses multiple antennas with different, for example perpendicular, polarizations to transmit or receive radio frequency energy. Pattern diversity uses multiple antennas, each having a unique radiation pattern, to transmit or receive radio frequency energy. One technique for controlling the radiation pattern of a particular antenna is to locate passive, or parasitic, elements at specific locations and orientations relative to the antenna. The passive elements absorb and re-radiate electromagnetic energy, acting to reflect, direct, or otherwise shape or focus the antenna radiation pattern in a desired fashion.
Traditional approaches to providing polarization and pattern diversity require antenna systems with multiple, independent antennas, which require additional space and detract from compactness. Moreover, to satisfy performance requirements of each antenna, additional structures, for example Reflectors, Directors, and Baluns, are typically provided to facilitate adequate operation of each antenna. This can pose a problem in designing an antenna system that simultaneously satisfies both compactness and performance requirements.
There are several examples of prior art that attempt to provide antenna diversity while retaining compactness of the antenna system. For example, U.S. Pat. No. 5,532,708 discloses a single compact antenna element comprising a “U” shaped body topped with a split crosspiece. The structure can be used in two modes. By supplying radio frequency (RF) energy to the bottom of the “U” shaped body, the structure can be made to behave as a monopole with a vertical polarization; by grounding the bottom of the “U” shaped body and energizing the crosspiece with RF energy, the structure can be made to behave as a dipole with a horizontal polarization, supported by a Balun structure which enhances antenna performance by providing isolation between the antenna and its transmission line. The antenna system therefore provides for sequential polarization diversity using few elements. However, since only one mode can be used at a time, the diversity capability of this antenna system is limited.
As another example, U.S. Pat. No. 7,215,296 discloses an antenna system that provides pattern diversity within a compact structure. A number of monopole antennas with the same polarization are arranged on a planar surface around a common reflector body that electromagnetically isolates the antennas from each other while also acting as a reflector for each antenna. Providing a common reflector for all antennas, as opposed to providing a separate reflector for each antenna, reduces the space requirements and manufacturing cost of the antenna system. However, as all antennas have the same polarization, this antenna system does not provide for polarization diversity.
Polarization and pattern diversity are important strategies for achieving performance requirements of many antenna systems. However, standard techniques providing for polarization and pattern diversity may result in an unacceptably large or complex system of antenna elements. Known antenna systems that attempt to provide for antenna diversity in a compact package have significant limitations with regard to antenna diversity. Therefore there is a need for a compact antenna system which can exploit polarization and pattern diversity by providing for multiple, simultaneously operable antenna elements with low complexity and a small number of components.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.